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Hydration can have a significant impact on the reading, so readings need to be taken at the same time of day with the same level of hydration. The [[Book Review - The 4 Hour Body|4 Hour Body]] recommends drinking 1.5 liters (3 pints) of water, waiting 30 minutes, urinating and then taking a measurement. I have found that with the whole body scales I can simply measure when I wake in the morning and I get a consistent enough reading to not require this hydration procedure. The biggest problem for an athlete is that BIA is particularly inaccurate when fat free mass, including [[Glycogen]], changes<ref name="BIA1"/><ref name="BIA2"/>. I've noticed that the day after a long run, the scales claim a much larger loss of body fat than is reasonable. Studies have shown that obesity is underestimated with many body fat scales.
==Optimizing Accuracy==
My approach to optimizing my scales accuracy is average and calibrate. My Tanita BC-601 scales offer several activity levels, so I find the level that matches a more accurate body fat measurement such as a [[DEXA| DEXA Scan]]. I then use an average of a week's readings to even out any hydration errors.
==Posture and Accuracy==
With BIA scales it's important to get the posture just right, as even small variations in the position of your arms can make a significant change to the reading. You must hold the handset with your arms at 90° with your elbows straight.